Facile immobilization of ethylenediamine tetramethylene-phosphonic acid into UiO-66 for toxic divalent heavy metal ions removal: An experimental and theoretical exploration

By the facile immobilization of ethylenediamine tetramethylene-phosphonic acid (EDTMPA) onto the surface and into the defects of UiO-66, a stable and efficient adsorbent named UiO-66-EDTMPA was obtained for the first time. In terms of removing aqueous heavy metal ions (Pb²⁺, Cd²⁺, Cu²⁺), the maximum adsorption capacities of UiO-66-EDTMPA reached 558.67, 271.34 and 210.89 mg/g, which were 8.77 (Pb²⁺), 5.63 (Cd²⁺) and 5.19 (Cu²⁺) times higher than raw UiO-66 respectively. The adsorption behavior of three heavy metal ions on UiO-66 and UiO-66-EDTMPA were investigated and compared through batch control experiments and theoretical studies. The main factors on adsorption progress (i.e., the dosage of EDTMPA, pH, ionic strength, co-existing ions, initial concentration, contact time, temperature) were explored, and the critical characterization (i.e., SEM, TEM, XRD, FT-IR, TG-DTG, XPS, N₂ adsorption-desorption test) were performed. Molecular dynamics (MD) simulation (radial distribution functions (RDF) and mean square displacement (MSD)) were also applied to reveal the adsorption behavior. Besides, two new quantum chemical analyses (Hirshfeld surface and independent gradient model (IGM)) were introduced into the interaction analysis between UiO-66 and EDTMPA. The complete results showed that (1) where the hydrogen bond and (vdW) connect EDTMPA to UiO-66. (2) The coordination between O, N atoms of EDTMPA and heavy metal ions (Pb²⁺, Cd²⁺, Cu²⁺) resulted in spontaneous adsorption. (3) The adsorption behavior agreed with Langmuir and pseudo-second-order model, endothermic reaction. In addition, the desorption and reusability study showed promising stable and sustainable performance. This work has some guiding significance for the experimental and theoretical study of removing heavy metal ions from aqueous solutions by MOF or modified MOF materials.